Querying documents in MongoDB is an essential aspect of manipulating and managing data. Document querying is carried out using evaluation operators that allow you to filter and select data efficiently. This chapter will delve into how to perform document queries using evaluation operators in MongoDB.
Firstly, it is important to understand what evaluation operators are. They are tools that allow you to perform specific operations during document queries. In MongoDB, there are several evaluation operators, including $eq (equal), $gt (greater than), $gte (greater than or equal), $in (in), $lt (less than), $lte (less than or equal ), $ne (not equal), $nin (not in) and others.
Let's start with the $eq operator which is used for equality. This operator compares the value of a field with a specified value. For example, to find all documents in a 'users' collection where 'name' equals 'John', you would use the following query:
db.users.find({name: {$eq: 'John'}})
The $gt operator is used to find documents where the value of a field is greater than a specified value. For example, to find all documents in a 'products' collection where the 'price' is greater than 100, you would use the following query:
db.products.find({price: {$gt: 100}})
The $in operator is used to find documents where the value of a field is equal to any value in a specified array. For example, to find all documents in a 'users' collection where the 'name' equals 'John' or 'Sarah', you would use the following query:
db.users.find({name: {$in: ['John', 'Sarah']}})
Evaluation operators are not limited to being used individually. They can be used together to form more complex queries. For example, to find all documents in a 'products' collection where the 'price' is greater than 100 and less than 200, you would use the following query:
db.products.find({price: {$gt: 100, $lt: 200}})
In addition, MongoDB also supports the use of logical operators in queries, such as $and, $or, $not and $nor. These operators allow you to combine multiple query conditions. For example, to find all documents in a 'users' collection where the 'name' equals 'John' and the 'age' is greater than 30, you would use the following query:
db.users.find({$and: [{name: 'John'}, {age: {$gt: 30}}]})
In summary, querying documents in MongoDB is a powerful and flexible task that can be accomplished in a variety of ways using evaluation operators. These operators allow you to efficiently filter and select data, making MongoDB a valuable tool for data management.
This was just a brief summary of the evaluation operators available in MongoDB. There are many other operators that can be used to perform more complex and specific queries. Therefore, it is important to understand and practice using these operators to become proficient at querying documents in MongoDB.
